The semiclassical instanton estimate of the thermal transmission probability for barrier crossing reactions as derived by the WKB approximation has a well-known deficiency. It diverges at the crossover temperature, between below the barrier and above the barrier pathways. A solution to this problem has been presented by using the uniform semiclassical energy dependent transmission coefficient as derived by Kemble instead of the primitive semiclassical form .The resulting thermal transmission coefficient does not diverge at any crossover temperature and the crossover temperature between tunneling and thermal activation is doubled. However, there remains a difficulty with the uniform semiclassical result of Kemble. It predicts that the half-point, defined as the energy at which the energy dependent transmission probability equals 0.5 occurs always when the energy equals the barrier height, which is incorrect as exemplified for an Eckart barrier. This may be corrected by modifying the Kemble form, either by shifting the energy or by adding a constant term to the Euclidean action, such that the resulting thermal transmission coefficient gives the correct leading order term upto the fourth power of the reduced Planck’s constant, as derived by Pollak and Cao. Shifting the energy is equivalent to a modified vibrational perturbation theory (mVPT2 and mSVPT4). Shifting the action by a constant generalizes a correction introduced by Eckart and later modified by Yasumori and Fueki (mYF). The mVPT2, mYF and mSVPT4 theories have been applied to various 1D model systems and the results are promising. The mVPT2 and mYF theories have been successfully generalized to multidimensional systems and applied to calculate thermal rates for the collinear H+HH and D+HH reactions.